Understanding the iodine cycle is important for three main reasons:

Iodine is an essential human micronutrient and a lack of it can lead to goitre and brain damage. Sources of iodine in the diet include seafood, but also crops grown on land in iodine containing soils – iodine is transferred to the land from the oceans via the atmosphere.

Iodine also has an important role to play in the chemical reactions that occur in marine air, being involved in ozone depletion and particle formation in the lower atmosphere. These reactions can influence local and global climate.

Finally, radioactive isotopes of iodine are released to the sea and air by the nuclear industry; we need to understand where they end up and how they might enter the food chain.

For a summary of the iodine cycle see Alex Baker’s iodine pages http://www.uea.ac.uk/~e780/airseaiod.htm.

Marine algae (seaweeds and microscopic phytoplankton) are thought to have an important role in the biogeochemistry of iodine. They can take up iodine and concentrate it in their tissues, for example the kelp Laminaria digitata can contain as much as thirty thousand times more iodine than the surrounding seawater¹. Algae are also known to be involved in the production of volatile of forms of iodine that can pass from seawater to the atmosphere, where they take part in the reactions mentioned above.

My PhD research is focused on how algae affect the chemical forms (species) of iodine present in the ocean. Most iodine is found as iodate (IO₃^-), however in surface waters significant amounts of iodide (I^-) are found². According to thermodynamic principles, iodide is unstable and should not be formed in seawater, so how did it get there? Some research suggests that algae are responsible³.

To investigate this further, I have grown cultures of marine phytoplankton in the laboratory and monitored changes in iodide and iodate. Initial results suggest that phytoplankton do indeed have the capability to convert iodate to iodide, but that this varies widely with species.

Results from the laboratory will be complimented by a study of iodine speciation in the Southern Ocean, around the Antarctic island of South Georgia. Samples were collected in December 2004 during a British Antarctic Survey research cruise aboard the RRS James Clark Ross.

Understanding the link between iodine speciation and biological activity could lead to the use of iodide as a tracer of productivity⁴. Additionally, the interconversion of iodine forms may also have a bearing on rates of production of volatile iodine species. Further experiments are planned to look at the interplay of iodide, iodate and organic forms, including volatiles, in phytoplankton cultures and natural waters.

My PhD supervisors are Gill Malin, Tim Jickells and Alex Baker and I am supported by a NERC studentship.

References:

1. Gall, E. A., Kupper, F. C. & Kloareg, B. A survey of iodine content in Laminaria digitata. Botanica Marina47, 30-37 (2004).

2. Campos, M., Farrenkopf, A. M., Jickells, T. D. & Luther, G. W. A comparison of dissolved iodine cycling at the Bermuda Atlantic Time-Series station and Hawaii Ocean Time-Series Station. Deep-Sea Research Part Ii-Topical Studies in Oceanography43, 455-466 (1996).

3. Wong, G. T. F., Piumsomboon, A. U. & Dunstan, W. M. The transformation of iodate to iodide in marine phytoplankton cultures. Marine Ecology-Progress Series 237, 27-39 (2002).

4. Tian, R. C. et al. Iodine speciation: A potential indicator to evaluate new production versus regenerated production. Deep-Sea Research Part I-Oceanographic Research Papers43, 723-738 (1996).